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Chapter 4 The Nature of Resources and the Resources of Nature • 71ents, however, are used up, transformed first into the pizza itself, then rapidlythereafter into waste. The cook and kitchen are not physically embodiedin the pizza, but the raw ingredients are. Thousands of years ago,Aristotle discussed this important distinction and divided causation (factors)into material cause, that which is transformed, and efficient cause, thatwhich causes the transformation without itself being transformed in theprocess. Raw ingredients are the material cause, and the cook and kitchenare the efficient cause.Other differences between these factors of production also exist. If wehave enough raw ingredients to make 1000 pizzas, those ingredientscould be used to make 1000 pizzas in one night, or one pizza a nightfor 1000 nights (assuming the ingredients were frozen and wouldn’tspoil, and we had enough cooks and kitchens). The economy can usethe existing stock of raw materials at virtually any rate, and time is nota factor. The productivity of raw ingredients is simply measured as thephysical number of pizzas into which they can be transformed. In addition,as the ingredients for a pizza are produced over time, those ingredientscan be used when they are produced, or stockpiled for future use.In contrast, while a cook or a kitchen may be capable of producingmany thousands of pizzas over the course of their lifetimes, they canproduce no more than a few pizzas in any given evening, even if limitlessingredients are available. The productivity of cooks and kitchens ismeasured as a number of pizzas per hour. However, this productivitycannot be stockpiled. For example, if we rest a cook for 6 nights, his capacityto produce a week’s worth of pizzas cannot be used up all on theseventh night.Georgescu-Roegen used the terms “stock” and “fund” to distinguish betweenthese fundamentally different types of resources. A stock-flow resourceis materially transformed into what it produces. A stock canprovide a flow of material, and the flow can be of virtually any magnitude;that is, the stock can be used at almost any rate desired. Time does notenter into the equation, so the appropriate unit for measuring the productionof a stock-flow resource is the physical amount of goods or servicesit can produce. Further, a flow can be stockpiled for future use.Finally, stock-flow resources are used up, not worn out. A fund-service resource,in contrast, suffers wear and tear from production but does not becomea part of (does not become embodied in) the thing produced.Instead, a fund provides a service at a fixed rate, and the appropriate unitfor measuring the service is physical output per unit of time. The servicefrom a fund cannot be stockpiled for future use, and fund-service resourcesare worn out, not used up. 17 Note that the classification of a17 Georgescu-Roegen, The Entropy Law, op. cit.